This invention relates to a getter assembly used in a vacuum apparatus, and, more particularly, to a getter assembly having a porous metallic support.
Many infrared and other types of sensors operate most efficiently and reliably when cooled to a cryogenic temperature, such as about the boiling point of liquid nitrogen, 77K, and are operated in a vacuum to provide thermal insulation for the sensor and to avoid condensation of materials such as water on the sensor. To effect these conditions in service, the sensor is mounted within an evacuated dewar/vacuum enclosure. The dewar/vacuum enclosure typically includes an insulated vacuum housing having a window through which the sensor views an external scene.
The interior of the dewar/vacuum enclosure must remain evacuated and at low pressure before and during service. The enclosure is initially evacuated during manufacture, and thereafter sealed. However, there is a continuous small outgassing of the structure inside the enclosure. To obviate the resulting small increase in gas pressure, getter material is placed inside the vacuum package and activated, effectively forming a chemical vacuum pump. A getter is a material that, when activated, captures gas molecules in the vacuum. The getter absorbs, adsorbs, and/or physically entraps oxygen and other molecules that are outgassed from the interior of the vacuum enclosure over the life of the product.
The getter material is available from commercial sources as a porous sintered ceramic mass that fits inside the enclosure. Due to its sintering method of manufacture, the dimensional tolerances of the sintered getter material are rather large, so that the getter material is not accurately sized to fit into the precisely sized enclosure. The dimensional irregularities would not otherwise create a problem, except for the fact that, after positioning inside the enclosure, the getter material must be heated to an elevated activation temperature falling within a relatively narrow temperature range in order to effect activation. If the temperature reached during the activation heating is too low, the getter material is not properly activated and is not fully effective. If the temperature reached during the activation heating is too high, the getter material may further sinter with an associated reduction in porosity that also leads to a loss of effectiveness.
The inventors have found that heating the getter material in vacuum to a precise activation temperature range in these circumstances is difficult, because the primary heat flow path into the getter material is thermal conduction from the walls of the vacuum enclosure. Due to the dimensional irregularities resulting from the manufacturing method, the getter material does not fit closely against the interior walls and the heat flow path is ill defined and irregular, leading to uncertainty as to whether the required activation temperature is reached in the getter material during a standard heating procedure.
The irregular heating of the getter material during the activation procedure cannot be compensated for by calibration or related techniques, because each piece of getter material is differently dimensioned and consequently has different heat flow properties during heating. The getter material cannot be forced against the inner wall of the vacuum enclosure too tightly so as to achieve good thermal contact, because ceramic particles may be rubbed away from the getter material to reside in the interior of the sealed vacuum enclosure. Such ceramic particles may find their way to the sensor surface and interfere with its sensing function during service.
There is a need for an improved approach to the use of a getter in vacuum packages that results in more certainty in the activation procedure, does not risk contaminating the sensor, allows the getter to operate properly, and is economical. The present invention fulfills this need, and further provides related advantages.